The present invention relates generally to an image output apparatus for printing an image produced by a printer, a facsimile or the like as a set of points (pixels), and more particularly, to an apparatus for performing image processing to represent a continuous halftone image in combination of a laser pulse width modulation (PWM) with a clustered-dot dither screening method.
Conventionally, an increase in the number of tone levels in halftone images produced by a laser printer has often relied on a method of subdividing each one pixel by means of laser pulse width modulation (PWM).
However, a reduced width of a laser pulse often results in variations in resulting products, particularly in the charging characteristics of a photoconductor and a toner, and in higher susceptibility to instabilities due to environmental variations such as temperature and humidity, thereby causing a problem of instable halftone reproduction in a highlight area, in which the image density is low, and a failure in providing a tone density commensurate with the number of divided laser pulses due to delayed activation of the laser and a delayed response of the photoconductor.
As a prior art approach addressing such problems, an article entitled “New Halftone Screening Technology Focused on Highlight Image Reproduction” (Japan Hardcopy '95 Transactions, pp. 143-146, by Yasuhiro Oda et al.) discloses a method of avoiding the use of narrow laser pulses by switching the halftone processing from line screen method to clustered-dot dither screen method only in a highlight area. JP-A-9-331448 discloses a method of stabilizing a reproduced image by adding adjustments to PWM outputs, using a look up table (LUT), corresponding to variations in environmental conditions and so on. Further, U.S. Pat. No. 5,134,495 (corresponding to Published Japanese Translation of PCT International Publication for Patent Application No. JP-A-6-504004) discloses a technology of performing a resolution transformation for increasing the resolution of a laser printer beyond the inherent engine performance by the use of multiple exposure of laser light which does not exceed a pixel threshold value.
Among the prior art approaches mentioned above, the method of processing a highlight area as clustered-dot dither screen is effective in stabilizing the reproduction of the tone in the highlight area, but may cause coarse isolated clustered-dot to be emphasized when a printer engine inherently has a large minimum pixel, leading to prominence of noise in the highlight area.
Also, when a look up table is relied on to perform a feedback to the varying environment and aging changes, as disclosed in JP-A-9-331448, a large look up table must be prepared or any interpolation means for continuously supporting the varying environment must be used in combination in order to finely respond to the varying environment. Otherwise, this method would emphasize instable images at boundaries at which one look up table is switched to another. Of course, simple modifications of PWM output values using look up tables without performing the feedback with respect to the environmental conditions would not present characteristic differences from the γ-correction which has been conventionally performed commonly on the halftone processing.
On the other hand, an instable region in a highlight area resulting from a shallow potential of an electrostatic latent image due to subdivided laser pulses extends as an instable potential region increases near toner attaching limits in the latent image potential. Consequently, as such an instable region in the highlight area extends, the feedback correction using look up tables alone will experience difficulties in achieving sufficient stability due to the influence of variations in individual photoconductor, the characteristic distribution on the particular photoconductor, and so on, in addition to the influence of temperature and humidity. For this reason, the formation of a small, deep electrostatic potential to suppress the generation of instable potential regions plays a critical role for stabilizing a highlight image quality.
However, a method relying on the resolution transformation as shown in U.S. Pat. No. 5,134,495 substantially requires pixel value information for interpolated pixels exceeding the device resolution, as it describes “using a raster of pixels to represent said desired image, said raster having a predetermined raster resolution, said raster resolution being different from said device resolution.” Thus, this type of method results in an increased cost of memories and associated circuits, as well as a longer data transfer time.
In addition, even though the resolution transformation functions as an effective means for a high end machine which exhibits relatively stable characteristics in terms of the photoconductor and so on, this is not true in low-price machines for use by general public, in which these characteristics are relatively instable, because the reproduction of isolated points is still instable as compared with dots of the inherent resolution originally provided by a printer engine. Thus, the resolution transformation is not suitable for the reproduction of thin lines, and is only effective in a halftone representation in which a plurality of dots are grouped into a set, as is the case with the clustered-dot dither processing, and in a relationship between clusters and dots as the smoothing of edges in fonts and so on.
It is therefore an object of the present invention to provide an image processing apparatus for use also in low-price laser printers that may exhibit instable characteristics, which implements the halftone processing for stably increasing a concentration of highlight tone by simple circuits, which can be mounted in ASIC, and accomplishes high speed and high quality image processing at a low cost.